The present invention broadly relates to a method of improving the stress corrosion resistance of high-strength, heat-treatable alloys. It particularly relates to a method of improving the stress corrosion resistance of martensitic stainless steels for use as bearing materials in high speed rotating machinery.
There are many instances in which structures or machines are built where some of the components are in a highly stressed condition. This stress can result in a phenomena generally referred to as stress corrosion which can produce cracks in those components possibly leading to failure of the component. This problem is particular acute in the bearing assemblies used for equipment which operate at high rotational speeds. For example, the turbopumps used with rocket engines can operate at speeds in excess of 100,000 RPM. Obviously in such applications, any bearing failure could result in catastrophic failure of not only the pump but also the engine and the vehicle to which it was attached.
During operation of a pump or other device rotating at high speeds, centrifugal forces act on the inner race of the bearing in a radially outward direction. If such forces become too great, the inner race expands radially moving away from the shaft on which it's mounted toward the outer race and reduce the clearance for the bearings. The reduction in clearance would increase the load on the bearings, cause overheating and possible bearing failure. In a similar manner, any clearance between the inner race and the shaft could result in galling or vibration, either of which also could lead to a failure.
To offset the effect of centrifugal forces, it is customary to size the inside diameter of the inner race slightly smaller than the outside diameter of the shaft upon which it is to be mounted. Assembly generally is accomplished by freezing the shaft to cause it to shrink and optionally heating the bearing to expand it. After the bearing is placed on the shaft and they both return to ambient temperature, the result is a so-called "shrink fit". The amount of shrink, among other things, will have a direct effect on the maximum rotational speed at which the assembly can be operated. The amount of shrink fit also is directly related to the amount of stress that will be placed upon the inner race of the bearing following assembly.
The susceptibility to stress corrosion of the material from which the inner race is formed in turn limits the amount of shrink fit that may be utilized. Obviously, any improvement in the stress corrosion resistance of an alloy would be beneficial in that it would permit a greater amount of shrink fit and allow operation at higher rotational speeds.